Novel Dynamical Magnetoelectric Effects in Multiferroic BiFeO3

Abstract

An atomistic effective Hamiltonian scheme is employed within Molecular Dynamics simulations to investigate how the electrical polarization and magnetization of the multiferroic BiFeO 3 respond to time-dependent ac magnetic fields of various frequencies, as well as to reveal the frequency dependency of the dynamical (quadratic) magnetoelectric coefficient. We found the occurrence of vibrations having phonon frequencies in both the time-dependency of the electrical polarization and magnetization (for any applied ac frequency), therefore making such vibrations of electromagnonic nature, when the homogeneous strain of the system is frozen (case 1). Moreover, the quadratic magnetoelectric coupling constant is monotonous and almost dispersionless in the sub-THz range in this case 1. In contrast, when the homogeneous strain can fully relax (case 2), two additional low-frequency and strain-mediated oscillations emerge in the time-dependent behavior of the polarization and magnetization, which result in resonances in the quadratic magnetoelectric coefficient. Such additional oscillations consist of a mixing between acoustic phonons, optical phonons and magnons, and reflect the existence of a new quasiparticle that can be coined "electroacoustic-magnon". This latter finding can prompt experimentalists to shape their samples to take advantage of, and tune, the magnetostrictive-induced mechanical resonance frequency, in order to achieve large dynamical magnetoelectric couplings.

The Experimentalist's Guide to the Cycloid, or Noncollinear Antiferromagnetism in Epitaxial BiFeO3

“…The analytical form of the H eff model has been consistently developed throughout its use in the literature to capture a more complete picture of the magnetic energy balances in BFO. [ 102,106,108–110 ] In the model used to accurately describe the cycloid characteristics, the total free energy is defined as [ 106,111,112 ] where u i is the unit cell's local soft mode, η is the strain tensor in the system, ω i is the AFD pseudovector, and m i is the magnetic dipole moment centered on Fe site i . The couplings between AFD, FE, and strain components are addressed in the first term, [ 108 ] and the magnetic couplings arising from the second term are considered to be …”

Mining for ideas

“…In an effort to find such a material that can be magnetised by electric fields, Omid Sayedaghaee and colleagues at Arkansas University, US, investigated the properties of bismuth ferrite (BFO), a material known to interact both with magnetic and with electric fields. 7 Using computer simulations, the researchers identified conditions likely to enhance BFO's electromagnetic response, improving its suitability for applications in fields like data storage.…”

Differentiated Strain-Control of Localized Magnetic Modes in Antidot Arrays

“…These include the exploitation of magnetic skyrmions 1,2 , magnetic memories 3,4 , heat assisted magnetic recording 5 or magnonic systems [6][7][8] . One of the challenges is to control the magnetic properties in a reversible way with low energy consumption, especially their dynamic properties for high-frequency systems 9,10 . This can be achieved by using electric or elastic-strain fields (magnetoelastic effect) instead of using a magnetic field 11,12 .…”

“…Nowadays, several avenues are being explored to exploit the unique properties of these materials. These include the exploitation of magnetic skyrmions, , magnetic memories, , heat-assisted magnetic recording, or magnonic systems. − One of the challenges is to control the magnetic properties in a reversible way with low energy consumption, especially their dynamic properties for high-frequency systems. , This can be achieved by using electric or elastic-strain fields (magnetoelastic effect) instead of using a magnetic field. , These effects have been widely reported in the literature, typically for continuous thin films on ferroelectric substrates. − However, the magnetoelastic field obtained for continuous films is generally homogeneous, which makes it difficult to spatially control differently the spin-wave energies …”